lang

Documentation

Attempting to install the relevant ISO 2- and 3-letter
codes as the enumerated possible values is probably never
going to be a realistic possibility. See
RFC 3066 at http://www.ietf.org/rfc/rfc3066.txt and the IANA registry
at http://www.iana.org/assignments/lang-tag-apps.htm for
further information.
The union allows for the 'un-declaration' of xml:lang with
the empty string.